As shown in FIG. 8, a heart 71 has a function of a pump that sends blood circulated through the body and flowing from superior and inferior vena cava 72, 72 into a right atrium 73R from a right ventricle 74R by way of a pulmonary artery 75 to right and left lungs and then delivers blood circulated from the lung by way of pulmonary veins 76 - - - to a left atrium 73L from the left ventricle 74L by way of an aorta 10 to the whole body.
For example, an aortic valve 11 is a cardiac valve that is formed at the exit where the blood flows out from the left ventricle 74L to the aorta 10, in which cup-shaped three valve cusps 12, - - - of a thin membrane are present as shown in the schematic view of FIG. 9.
In each of the valve cusps 12, a portion defined along a commissure portion K of a cusp to a valve annulus on one end of a free margin F as an ON-OFF end→a bottom end CL of a coaptation zone just therebelow→a bottom B of the valve cusp→a bottom end CL of the other coaptation zone→a commissure portion K of a cusp to a valve annulus on the other end is integrated with the inner wall as valve annulus of the aorta 10 to constitute a fixed edge 13.
Then, along with dilation and contraction of the heart 71, since the pressure in the left ventricle 74L becomes negative during diastole, coaptation zones Zc formed on the side of the free margin F of the valve cusps 12 are jointed to each other to close the valve 11 as shown in FIG. 9(a), whereas since the pressure in the left ventricle 74L becomes positive during cardiac systole, the coaptation zones ZC of the valve cusps 12, 12, - - - are spaced apart from each other to open the valve 11 as shown in Fig. (b).
However, when atheriosclerosis, etc. proceed along with aging and calcium is deposited to calcify and harden the valve cusps, the valve cusps are restricted in their movement and do not open sufficiently, causing disease such as aortic valve stenosis.
According to the recent study, from 2 to 3% of the population of 26,000,000 aged 65 years or older in Japan suffer from the disease.
When the aortic valve stenosis is caused, since pressure loss is generated at the affected area, the left ventricle intends to maintain the blood pressure in the aorta at a normal 100 to 120 mmHg (during cardiac systole) contracts at a pressure higher than that (for example, 150 mmHg).
As a result, the left ventricle wall becomes thickened and causes hypertrophy and, when the thickness of the wall of the cardiac muscle is excessively increased, since the blood flow supplied from the coronary arteries is no more sufficient, the heart enters a hypoalimentation and insufficient oxygen state (miocardial ischemia). Particularly, if myocardial ischemia is caused when the oxygen demand is increased, for example, during activity, it may possibly cause symptoms such as chest pain and syncope.
In a case where the blood pressure difference exceeds, for example, 50 mmHg, this is judged as an indication that surgery is needed.
The most popularized treatment is valve replacement by using a prosthetic valve.
The prosthetic valve includes two types, that is, a mechanical valve formed of carbon, graphite, etc. and a heterogenic biovalve prepared from biomembranes of animals such as swinery and cattle and they are operated by the procedures as shown in FIG. 10.
That is, FIG. 10 is an explanatory view showing the outline of prosthetic valve replacement. As shown in FIGS. 10(a) to 10(b), all three valve cusps (left coronary cusp, right coronary cusp, noncoronary cusp) 79, - - - of the aortic valve situated at the inlet of the aorta 10 are excised while leaving only the valve annulus in contiguous from the left ventricle 74L to the aorta. Then, as shown in FIG. 10(c), the inner diameter of the aorta is measured by a sizer 81 as shown in FIG. 10(c) (refer to Patent document 1) and a prosthetic valve 82 having a valve annulus outer diameter equal with the inner diameter of the aorta is stitched to replace the valve.
[Patent Document 1] JP-T No. 2000-502937
However, even when replacement is conducted by the prosthetic valve 82, since the prosthetic valve 82 has a shape in which the valve cusps 84 - - - are attached to a valve annulus 83, when it is stitched to the aorta 10, the inner diameter of the aorta 10 is decreased by so as much as the thickness of the valve annulus 83 and the portion forms an orifice, causing pressure loss.
For example, in a case where the inner diameter of the aorta 10 is 20 mm, when a prosthetic valve 82 having the valve annulus 83 of 3 mm thickness is mounted as the prosthetic valve, the inner diameter of the prosthetic valve 82 is decreased to 14 mm even when it is opened fully and the area ratio is decreased to about 1/2. Then, a pressure loss is caused to the portion and although the valve is replaced with an aim of decreasing the pressure loss caused by aortic stenosis, the loss cannot be decreased sufficiently.
In a case of using a mechanical valve as the prosthetic valve 82, it is necessary to continuously take an anti-coagulant agent such as warfarin for for one's whole life, which results in a problem that it is difficult to arrest the bleeding in a case of injury, as well as, it is also pointed out a risk that teratogenesis may be caused possibly to a fetus when a pregnant woman takes the agent.
On the other hand, there is no requirement of taking warfarin in a case of using a heterogenic biovalve, but this involves a problem that the cost is as high as 1,000,000 yen or more per one valve unit.
Further, since both the mechanical valve and the heterogenic biovalve are foreign elements in the body, they may cause a risk of rejection.
Accordingly, as a result of an earnest study, the present inventor has established, instead of valve replacement, an aortic valve reconstruction of repairing an aortic valve by decalcifying hardened valve cusps by using a supersonic surgical aspirator, etc., thereby leaving valve cusps that can be used as much as possible, and forming only the valve cusp that is no more usable by using an autologuous biomembrane, etc.
In this case, when the valve cusp is formed by incising an autologuous pericardium, etc., since this comprises an autologuous biobody tissue, there is no problem of causing the rejection or blood clotting reaction (thrombus) and there is no requirement of purchasing an expensive prosthetic valve.
However, there has been a problem in this case how to cutout a valve cusp from a planar pericardium in the shape and the size conforming to the diameter of a patient's aorta.
It is considered that the diameter (or radius) of the valve annulus corresponds generally to the shape of the aorta. However, in a case of measuring the diameter, for example, by a sizer 81 used for attaching the prosthetic valve (refer to FIG. 10(c)), all valve cusps have to be excised. If a usable valve cusp is left, the valve cusp hinders measurement for the diameter of the aorta.
Accordingly, the present inventor has attempted to use a model for the shape of the valve cusp based on the peripheral length of the valve cusp as shown in FIG. 11.
At first, assuming that an aorta is a circular cylinder having an inner diameter (diameter) D and that the central angle for a free margin LF extending from one commissure portion K of a cusp to a valve annulus, passing through the center PF of the aorta to the other commissure portion K of the cusp to the valve annulus (central angle for the commissure portions K, K) is 120° in a valve-open state, the length of the free margin is equal to D.
On the other hand, when assuming that the height from one commissure portion K to the bottom end CL of the coaptation zone ZC where the valve cusps are joined to each other as h (usually the height is considered sufficient if h=10 to 12 mm), the coaptation zone ZC has a rectangular shape having a width LF (=D) and a height h.
Since the central point PC for the base B of the valve cusp is at a position lower than the bottom end CL of the coaptation zone, the length LB for the base of the valve cusp extending from the bottom end CL of one commissure portion K, passing along the base of the valve cusp through the central point PC thereof to the bottom end CL of the other coaptation zone is larger than the inner peripheral length of the aorta having the central angle of 120°.
Then, when the shape of the valve cusp is formed as a semi-circular-square shape comprising a semi-circular cusp base portion having a length LB thereof up one-half circumference (=πr) of the inner periphery of an aorta and a rectangular portion corresponding to a rectangular coaptation zone ZC in contiguous therewith while making an allowance for stitching formed by cutting out a pericardium conforming to the shape and it is used for animal experiment of forming the prosthetic valve cusp of an aortic valve, it showed good result after surgery.
In this case, if it is possible to easily measure the inner diameter D of the aorta in a state of excising only one valve cusp, it is extremely convenient since the aortic valve can be formed while preparing the valve cusp by cutting out the pericardium during surgery.